Technical Field of the Invention
This invention relates generally to multi-antenna arrays, and more particularly to adding pre-distortion in systems including hybrid digital/analog precoders.
Description of Related Art
Communication systems are known to support wireless and wireline communications between wireless and/or wireline communication devices. Such communication systems range from national and/or international cellular telephone systems, to the Internet, to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LIVID S), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera, communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver is coupled to one or more antennas, and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
Some RF transmitters, including those used in many Wi-Fi base stations, employ an antenna technology called multiple input/multiple output (MIMO) in which multiple antennas are used for sending and receiving more than one data signal simultaneously over the same radio channel by exploiting multipath propagation. In addition, beamforming techniques can be used to achieve spatial selectivity. In general, beam forming can be performed by precoding data streams before sending the data streams to an antenna array, which often includes hundreds of antennas.
The next generation (5G) architecture of cellular base station will transmit dozens of high capacity data streams to multiple users using MIMO techniques (MU-MIMO) by forming a dedicated beam towards each user. The beam forming is done by precoding each of the data streams prior to reaching the hundreds (N) of antenna elements.
Power consumption constraints on an MU-MIMO base stations will require transmit power amplifiers to become more efficient, that is be able to transmit linearly with higher power, but at the same time avoiding increasing power consumption. Unfortunately, power amplifiers generally have a non-linear response when operating at higher amplification levels, resulting in increased power consumption for a given transmit power at higher amplification levels.
It is known to add pre-distortion compensation into the digital transmit path of an RF transmitter, specifically by adding digital pre-distortion after an all-digital precoder, but before the digital to analog converters used to convert the digital signals for use in the analog transmit path. It will be appreciated that hybrid analog/digital precoders are often preferred over all-digital precoders for 5G and other systems with large antenna arrays, because hybrid analog/digital precoders generally use fewer RF chains, thereby avoiding energy consumption associated with mixed signal components, and can provide reduced baseband processing complexity. Unfortunately, for hybrid analog/digital precoders, digital pre-distortion cannot be efficiently implemented using the same techniques used for all-digital precoders.